Microlithography apparatus (pattern-transfer apparatus) employing a charged particle beam (e.g., electron beam) as an irradiation source are currently the subject of much research aimed at further decreasing the feature size of integrated circuits and the like to increase circuit density without sacrificing throughput. Such microlithography apparatus evolved from previous technology developed for such technical arenas as transmission electron microscopy and scanning electron microscopy. Thus, microlithography apparatus employing an electron beam, for example, employ electron lenses and deflectors to control the propagation of the electron beam in a usable way.
FIG. 1 shows an objective lens 10 of a conventional charged-particle-beam microlithography apparatus. FIG. 1 also shows a reticle 11 and a substrate (e.g., wafer) 12 situated relative to the objective lens. Trajectory paths of the charged-particle beam are denoted 13 and 14, wherein the beam of path 13 extends along the optical axis AX (parallel to the Z axis) of the objective lens 10 and the beam of path 14 is laterally displaced from the optical axis AX. With the beam of path 14, the lens 10 typically exhibits substantial aberration. The aberration is more pronounced with increasing lateral displacement of the charged particle beam from the optical axis.
To provide some correction of such aberrations, conventional charged-particle-beam microlithography apparatus employ supplementary lenses or supplementary deflectors to cause the charged-particle beam to propagate under conditions that are the same as if the beam were propagating on-axis. Such lenses include "variable-axis lenses" (abbreviated VAL) or "variable-axis immersion lenses" (abbreviated VAIL). In FIG. 1, lenses 15 and 16 are VAL supplementary lenses and lenses 17 and 18 are VAL deflectors. The supplementary lenses 15, 16 and the supplementary deflectors 17, 18 are typically energized to a degree that depends upon the amount of off-axis lateral displacement of the charged-particle beam. To such end, the supplementary lenses 15, 16 generate a field represented by the following Equation (1): EQU bz!=(x.sub.0.sup.2 +y.sub.0.sup.2)B"z!/4 (1)
The deflectors 17, 18 generate a deflection field represented by the following Equation (2): EQU (dxz!, dyz!)=(x.sub.0 B'z!/2, y.sub.0 B'z!/2) (2)
wherein Bz! is the distribution of the on-axis field of the objective lens 10, and (x.sub.0, y.sub.0) is the off-axis displacement, i.e., the location (when the optical axis is the origin in the incident plane of the at which lens) a charged-particle beam incident off-axis (beam of path 14) can pass through the objective lens 10 under nearly the same conditions as if the beam were incident on-axis. As a result, aberration is diminished. FIGS. 2(a)-2(c) are plots showing the distributions of Bz!, the first derivative B'z!, and the second derivative B"z! of Bz! with respect to z, wherein each horizontal axis is the Z axis and each vertical axis represents magnitude.
However, astigmatism remains a problem even though the charged-particle beam is deflected as described above.
A conventional way to address the astigmatism problem is to insert an astigmatism compensator separate from the deflectors 17, 18 and the supplementary lenses 15, 16. A conventional astigmatism compensator is normally configured as an octapole coil. Four poles of the coil form an X-direction astigmatism-correction coil, and the other four poles of the coil form a Y-direction astigmatism-correction coil. The magnetic field formed by such a coil in the X-direction is shown in FIG. 3. Specifically, for example, a magnetic field 25 is formed by the tetrapole coils 21-24. Deflection fields are formed at positions away from the axis, while the field on the axis of the compensator is zero. However, since the charged-particle beam is mostly off-axis during exposure, new aberrations are generated even though such an astigmatism compensator is used.